In electric motors used today, bearing seats, into which the roller bearings that receive the rotor of the electric motor are inserted, are provided in the housing and in the housing cap, also known as a motor end plate, that closes the housing. The roller bearings may be either ball bearings or roller bearings or combined axial/radial bearings, depending on the particular intended use of the electric motor. The provision of bearing seats in the housing or in the housing cap (motor end plate) of an electric motor makes stringent demands in terms of dimensional stability of the components and means relatively high production effort and expense to attain bearing of the rotor as concentrically as possible relative to the stator of the electric motor. Under the operating conditions of the electric motor, the rotor should have an air gap that extends as concentrically as possible relative to the stator bore. Upon assembly of the electric motor, the stator is as a rule shrink-fitted into the cylindrical housing. The fit between the housing and the housing cap (motor end plate) that closes it causes positional errors with respect to the rotor axis, which often necessitate an additional grinding out of the stator bore and subsequent trimming on a lathe of the outer diameter of the stator, in order to attain the requisite dimensional stability with respect to a concentric air gap between the outside of the rotor and the inside of the stator bore.
Postmachining of the stator bore or of the outer diameter of the rotor on the one hand represents further production effort and expense for bringing about the concentricity of the air gap between the rotor and the stator bore of an electric motor. In an electrical machine, on the other hand, the air gap has considerable influence on the resultant efficiency.
The provisions made to assure a concentric air gap between the outer diameter of the rotor and the inner diameter of the stator bore by additional grinding out or subsequent turning on a lathe do offer one possible way of making the air gap of an electrical machine concentric, but they represent an extremely unsatisfactory, time-consuming provision that is highly disadvantageous in large-scale mass production of electric motors.